Update dependency mocha to v8

Author: renovate-bot

package.json

@@ -22,7 +22,7 @@
   },
   "devDependencies": {
     "chai": "4.2.0",
-    "mocha": "7.1.1"
+    "mocha": "8.1.1"
   },
   "dependencies": {
     "bluebird": "^3.0.6"